Simple Mouth Rigging
Pretty much just start your bones at the edge of the mouth and work your way in, leaving a gap in the middle for size difference when the edges of the mouth move out.

I recommend using some sort of mouth animation reference sheet and choosing which shapes the mouth will be taking on most often, especially if your creature doesn't speak a human language where mouth movements are fairly non-caricatured and straight forward. This rigging system, however should do for a basis of most things, and with a little tweaking and weight-painting can be made much less scabby, however the immediate results aren't terrible :)

Obviously include more bones around areas that will need greater control over their deformation and do not forget about that gap in the middle of the lip.